This invention relates to monolithic ceramic capacitors having low-melting metal film electrodes that are buried in a dielectric ceramic body having a high-temperature-firing granular barium titanate phase and a low melting intergranular phase.
In ceramic bodies of this type, a sintering aid or low melting flux is employed. Most often the flux is a glass frit as is described in the patents to G. H. Maher, U.S. Pat. No. 3,885,941 issued May 29, 1975 and to I. Burn, U.S. Pat. No. 4,120,677 issued Oct. 17, 1978. The addition of such fluxes to the ceramic start materials has the highly beneficial results that the temperature at which the ceramic may be fired to maturity is reduced, lower cost buried electrodes of silver-containing and other low-melting metals may be co-fired without their melting and running out, and volatile elements such as lead, bismuth and cadmium may be more readily incorporated in the dielectric to achieve improved electrical properties.
However, increasing amounts of flux leads to a reduction in the overall dielectric constant (K) of the body because the volume of low K grain boundary phase increases. Heretofore, it has been necessary to include about 10 weight percent flux or glass with the ceramic precursor materials to achieve full densification of the body at temperatures under 1150.degree. C. But with so much flux added, the dielectric constant at about room temperature is usually less than 2000 and seldom as high as 4500.
It is known to add zirconates and/or stannates to barium titanate to lower the Curie temperature to a point near room temperature. Unfortunately, incorporation of zirconium and tin compounds in barium titanate raises the firing temperature required for full densification of the body. In the patent to G. H. Maher, U.S. Pat. No. 4,066,426, issued Jan. 3, 1978, it is taught that small additions of cadmium substantially reduce the amount of zirconate that must be included to achieve a given downward shift in the Curie temperature.
It is an object of the present invention to provide a monolithic ceramic capacitor capable of being fired at less than 1150.degree. C., wherein the ceramic body has a relatively high dielectric constant at near room temperature.
It is a further object of this invention to provide such a capacitor containing an amount of cadmium and donor ion that is optimum for bringing the Curie temperature down and maximizing the dielectric constant of the body at about room temperature.
It is a further object of this invention to provide such a capacitor wherein there may be substituted for some or all of the especially volatile and poisonous cadmium, other cations capable of entering the barium titanate crystal lattice on the large cation sites with a charge of one.